Present techniques for forming small features in the thin films in integrated circuit manufacture make use of etching in the presence of etch resistant layers or sections of photoresist that have been irradiated to provide a selected pattern. This involves traditional photo engraving or photolithographic processes. However, with the continued miniaturization of semiconductor integrated circuits it has become increasingly difficult to maintain the necessary resolution between lines and features which comprise the components of the integrated circuit.
Alternative methods, which are well known in the art, have been designated expandable mask methods, liftoff methods or stencil methods. However, because of the methods used for hardening the photoresist and then etching non-hardened areas, some of the deposited films have an undesirable negative slope or overhang which poses problems to subsequent processing steps. Similarly, some current wet chemical and high pressure plasma etch processes often produce unwanted lateral etching which undercuts the deposited film. As the size and geometries of the devices have continued to shrink, as well as the openings and lines therein, the necessity for closely spaced features and openings, and the necessity to accurately maintain a high degree of resolution, has made more acute the need for a thin film with an anisotropically hardened edge that is very well defined to produce small features in the masking layer.
The problem is highlighted with a photoresist film that is coated onto a non-planar substrate having small isolated features or a group of features that are bunched close together. The non-planar surface scatters the incident radiation used to harden the photoresist and may result in excessive film loss. Whether the undesired result is over-development, over-exposure, or any other baseline shifting, the features may be enlarged, resulting in a profile which is dramatically different from that desired. While the features may be widely spaced to obtain the proper resolution, the spacing is excessive and packing density suffers. Reactive ion etching and ion milling have been used to minimize standing wave patterns caused by interference from reflected light and to provide a well defined or step edge on the protective film, but this requires three level processing, namely, a thick organic film, an inorganic intermediate masking layer, and a thin top layer of X-ray or E-beam photoresist.
In view of the need for a method for providing very small features in a protective film used in integrated circuit manufacture, the need existed for a method for anisotropically hardening the film to define the desired edge.